Nowadays, integrated circuits (ICs) may comprise capacitive sensors such as a moisture-sensitive sensor, e.g. a relative humidity (RH) sensor or a liquid immersion detection sensor. The capacitive sensing element of such a sensor typically comprises a pair of electrodes, i.e. capacitor plates separated by a dielectric medium, wherein the dielectric constant of the dielectric medium is sensitive to a level of the analyte of interest, e.g. moisture. The analyte of interest is typically absorbed by the dielectric medium, which causes the dielectric constant of the dielectric medium to change, thereby changing the overall capacitance of the capacitive sensing element. Hence, the determination of the capacitance of the capacitive sensing element represents an indication of the levels of the analyte of interest to which the capacitive sensing element has been exposed.
Such sensors may be included in the IC design for a number of reasons. For instance, such a sensor may be included in the ICs to determine whether a malfunctioning IC that has been returned, e.g. to its manufacturer, has been damaged by exposure to moisture, e.g. an immersion event, or whether the IC itself is faulty. The determination of such external influences as a cause of malfunction may be of crucial importance to deciding whether or not the customer returning the IC or an electronic device including the IC is entitled to a warranty claim on the device, as misuse such as the aforementioned immersion event typically invalidates the warranty.
Alternatively, such a sensor may be part of the functionality of an IC. There is for instance a trend towards providing near-field communication ICs such as radio-frequency (RF) identification (ID) chips with a range of sensors, such as temperature sensors, ambient light sensors, mechanical shock sensors, liquid immersion sensors, humidity sensors, CO2 sensors, O2 sensors, pH sensors and ethylene sensors, which for instance may be used to monitor the ambient conditions of a product tagged with the chip such that product quality control can be achieved by monitoring the sensor readings of the chip.
It is particularly attractive to integrate at least some of these sensors in the back-end of the manufacturing process of an IC, such as in or on the metallization stack, as this facilitates a cost-effective route to such integration due to the fact that such integration can be achieved with minimal alteration to the process flow.
A drawback of such capacitive sensors is the relatively slow response time. There are two factors contributing to this slowness. Firstly, the limited diffusion rate of the analyte into the dielectric medium of the sensing element causes an intrinsic delay, and secondly the generated measurement signal is an analog signal, for which the conversion into the digital domain adds another processing step and an associated delay.